1. Field of the Invention
The present invention generally relates to a Write-Once type storage medium, such as a DVD-R (DVD-Recordable), which has much higher recording density than a known CD (Compact Disc). More specifically, the present invention relates to an apparatus for and a method of detecting, from a DVD-R, prerecorded information including address information needed to detect the recording position at the time of recording video information and rotation control information, such as synchronization information and wobbling information, used for the rotation control of the DVD-R (these prerecorded information will be hereinafter referred to as "pre-information").
2. Description of the Prior Art
Recently, there is known a CD-R (Compact Disc-Recordable) which has approximately the same recording capacity as a CD and is recorded with pre-information to enable additional recording thereon using the pre-information recorded. A CD-R is recorded with pre-information, at the pre-formatting stage of its manufacturing, in such manner that information tracks (i.e., groove tracks and land tracks) for carrying information is wobbled in a wave-like manner at the frequency obtained by frequency-modulating the pre-information to be recorded. When recording information such as video and/or audio information (these information will be hereinafter referred to as "main information", to distinguish them from the above-mentioned pre-information) onto a CD-R, the wobbling frequency of the wobbled tracks is detected to obtain the reference clock for the rotational control of the CD-R. Then, the drive signal for the rotation control of the spindle motor is produced on the basis of the reference clock thus obtained, and the recording clock including the timing information synchronized with the rotation of CD-R is also produced. Further, at the time of recording main information, the pre-information is reproduced to obtain the address information indicative of the address (i.e., position) on the CD-R and then the desired recording position of main information is detected using the address information thus obtained, thereby enabling recording main information.
However, since a DVD-R has a track pitch between neighboring tracks of approximately half of that of a CD-R for the reason of achieving high recording density, the wobbling signals of neighboring tracks interfere with each other and the wobbling frequency may not be correctly detected if the information tracks are wobbled like the manner of a CD-R. In this view, the information tracks of DVD-R (e.g., groove tracks) are wobbled at the frequency based on the reference clock to record pre-information, and pre-information is additionally recorded in the form of pre-pits formed on the tracks (e.g., land tracks) neighboring the information tracks. The pre-pits are formed uniformly over the all surface of the DVD-R so that the reference clock can also be obtained from the pre-pits if necessary. Conventionally, the pre-information is obtained by detecting the pre-pits, and the rotation control and the recording control is performed based on the pre-information thus obtained.
The pre-pits are detected in the following manner. First, a signal including the wobbling frequency component (hereinafter referred to as "wobbling signal") is obtained by irradiating a light beam onto the information tracks and then a gate signal synchronized with the wobbling signal is produced. Then, the wobbling signal is compared with a predetermined threshold signal to extract the signal component exceeding the threshold. Then, the logical product of the signal component thus extracted and the gate signal is taken to extract the pre-pit signal from the wobbling signal, thereby detecting the pre-pits.
Therefore, noise may be mixed into the wobbling signal and may be erroneously detected as the pre-pit if dust or the like is present on the disc or there is crosstalk intermixture from the neighboring tracks. Consequently, the address information or the synchronization information, necessary for the position detection at the time of main information recording, can not be correctly obtained, resulting in malfunction of the recording apparatus for DVD-R.